I think the only way to completely fail is to quit.
This is as close as we can get to a fountain of youth (at least aesthetically). Bodybuilders can hit their peak in their 30s and 40s, while guys who don't work-out have been going down hill since their 20s
well...
I don't know am I a bodybuilder... but I know for sure that if I continue to train hard long enough I don't suddenly lose my muscularity etc

, "oh yeah, just wait when you get kids/wife/career/life".... lol that won't happend

!
by the way, the fountain of youth rly is running/swimming

, but I get what you mean.